Method of avoiding distortion in synthetic resin sheet manufacture



Sept. 28, 1 54 J. L. CHYNOWETH METHOD OF AVOIDING DISTORTION IN SYNTHETIC RESIN SHEET MANUFACTURE Filegi April 25, 1951 IN VEN TOR. Jafm L. Cig rwwezfi BY A T T NE Y.

Patented Sept. 28, 1954 UNITED STATES BATFEN'E" team?- METHOD. OF AVOIDING: nlfimggugu SYNESHEEEQREfilN: QFHMAW, A 331W? John L, Chynoweth, Wilmington, Del assignortm E; I; du' Pont de Nemours':and:1 Company, Wile. mington, DeL, a corporationlefsnelamare App1i9e i9 -A9ril= 3, 1951, ekfitafififi? This invention relates to the. prgps gatign .oi synthetieresin. sheets andmore peytipul 11 t9 a continuous process of castinamfll yltnleth acrylate sheets.

TheR, T. Fields U. s. Patent, 2 5,37,g70, issu 4 January 16, 1951, describes a, pz o ces's, Q13, pply f meri ing methyl hecry e b s-t e? the mongmeric ester, usually containing sq po yme p n e un ace o a b dy, .01 nseque-t 11s salt so t on hi h is ubs anti l se urated, at. ratin mpe ature 'lihe. pl cfiefin i s qarried outlay initiating the polymerizatiqn ,qf, the mqnomer ester w t e fiiniqnergy though. he m l neney m v be used, iirfteng-v nexa ure are m ntain d e ow 80 C- Cl ar. pelym ri sheets of pl me hy me he r late, poly y ene em, ar nrd nuc t en ms solutions of the preferred salts, such as meg-,1 nesium. chloride a i h u qhlqride ar ufiQQ 1.1 Fie d ro e i o step in s yaeee over nevio s techniq es of prl du ine sq merie sheeting uch as t at, described in th 39.1 1;v a1. S. Patent 2, 154,fi39, in .vglgieh pply me iat qn s car ied out in molds. r 1 we. nose of w heet io l s e r ted-b e qm press b e sket- B Qwins he ewe-semi the Fields patent supra. and he pmeees F ifie elo edth ant ase, super qr nq ymegip. re in s eet of op ica quality may be bbtamed-.

In car n ou h g e a e qfi n q fie 9i cen inuous ep ng he t 0 ymex e D m thy met ee y and o he .RQ WRSA num er at. p oblems; h be n. e qoqme Q 1%. pm. me, mper ectiem ree heets vpd: especieny nt qall p rfec he t w e 0 M thetdurine nq ymer ze n. oi: he: e e 9% mund, contact o that qm qued .efi me solid surfaces could not be tolepated Mql eqver. the u per exposed .sm ahc Q heet Q P0 merizine or anic compound mu P v ,.e et substantially th same. rate. as v he 1.9. ex Mem s; thereof and should n t. it as been. l i' b in. conta t with substa es, me ieh hit P W- m rizatiqn.

2, The invention relates" in general t0 the polyrneriga tion bf 'aL' lnonomerici drganic compound: by @smpueng 'a cplnpo'undes' a" liquid la y'ei' on" the slu' fgcebf'a bddy' f an aqueous salts'olu' tjgnengi therea ft efpol-ymerizing the compound atetemperature'betweefio and C., the aqu'e qus salt 'sb1ution"bein'g maintained between 29? Cl and"8 i CI, being'substantially sat' ux e teg with a, sla lt at said-"temperature 'and having at s peeifie g' relvit-yfft least als greatesthe pcg 1yr 1 e 1 being" formed; The polymerizatibn' proeess i s'cpndueteqi' ina gcer'd withthe invert-f 1319!}, by, cpntinudus ly ciisti'ibuting the "liquid polyr'nerizable orgahiQflcoQlbofiQijf upon "the y? q e {1 9m hP iii fi' e li m. while c min 'p "we; liqu'ia-" m u y m n Q embv' ple. elf f 'yer y el ml merizefi' e n, e e e System/55" 'e' new e on. h exp s d.u b'erb r q he v in igm .dur u po ymeri t bh "'e' HY main a ng n. seem: meeeebni 'mhi tp' ..n t e eeuebus. e j ut o l he eb e slid ei bbiect of h inven- -Q i l be m re ee'dily' "e der fi d' by re e n e t t e dxelwi le w i hTdi e' lm ti l i trate c mi 'of eme tetl l tha maybe 4. and by reel-r 9 t e ex mp e f l trafi nneiev zedt embedim HE o he n in F gu 1 is re. i l meti a P a w of an m na es 9. xm e @1115. h in en on J 1 ere sew p se nin t e. x le t j i e n e m u oi f surfeceqf tl lensatlt so utign'.

me e 2; s is at he apparatus. shown in Figuge. efi ss a wm F ure l ens. om

bie fiien tr whi h s Q Q-dWQ-S e1 e e iselt thro gh in et itsel ti 1 m w i '1 w e a, eto tr ,A ee? t l wlne we t i nuonsgly oyer the v(ileum .1 2; bein 'dish reed thr u h Pin 1- A s tup, o; ,m qme i an p l xmerime h me lat is, .9 d. @9 the suxiage r the @1 1t. l u n h n uel l belt .4, weed iromqr ether suitable lnateri the whee .0: the

runm 2 e9nq n w. the a u us e ee et pe- A eek of flue eweet i ht 'B equ ng 15m. 9 QWFHWEE in en i .1 ew m x met 00- eegs a fi s. ses msleq ew th east 2 1 5 1. e l v as theis um of ener 9 Q L t 'ne'PP Xm 5 9a.- ymn substan j el e mnlemn i mime-$1 zation, the finished sheet C is continuously withdrawn from the unit. The entire unit is closed to the atmosphere by covering the top of the casting tray with glass 5 or other suitable material, and the end of the unit, where the sheet is withdrawn, is sealed from the atmosphere by flaps 6 of rubber or other suitable flexible material which act as the exit seal. Prior to operation of the unit, the air in space D is replaced with nitrogen, or other inert gas, and during polymerization the inert gas is introduced into the sealed unit through header 1, the gas flowing over the substantially completely polymerized portion of the sheet to reduce the concentration of methyl methacrylate vapor. The gas leaves the unit through the exit seal under flaps 6.

The barrier used for restricting the lateral flow of the polymerizing resin on the concentrated salt solution may be a vertically disposed flexible belt, link belt or any other suitable retaining means. As shown in Figure 1, two endless belts are used, the belts being advanced at the desired rate by driven pulleys it, operated by a power source not shown. The belts are moved through a path in front of the Monel metal guides It and around idling pulleys [2. The monomer-polymer syrup flows over the salt solution, forms a layer confined by the belt 4, advances at substantially the same rate as the belt, polymerizes as it advances and as the polymerized sheet leaves the tray and the support of the salt solution held therein, it passes over idler rolls [3 and i4 and above the rubber exit flexible walls it.

The dotted line 8 in Figure 1 illustrates the approximate dividing line between substantially completely polymerized material and the polymerizing mass of material. It is important for optimum results, for reasons described hereinafter, that the inert gas be introduced into the unit at a point immediately beyond this dividing line on the side of the polymerized portion, the gas flowing in the direction designated by the arrows 9 and passing out of the unit around or under flaps 5. Since the up-stream end of the unit is sealed from the atmosphere, a sustantially stagnant space of inert gas and vapors of the polymerizable liquid is maintained over the liquid portion of the polymerizing mass. Hence, by introducing the inert gas into the unit at a point immediately beyond the dividing line between the polymerizing mass and the substantially solid polymerized sheet, the gas flows over the solid sheet to reduce the concentration of vapor of the polymerizable liquid in the inert gas.

It is important that the inert gas be introduced immediately beyond the division between the polymerizing mass and the substantially solid polymerized sheet so that the flow of gas is toward the exit end of the unit. If the gas were permitted to flow over the entire length of the unit, that is over both the liquid polymerizing mass and the substantially solid portion of the sheet, the current of gas would promote evaporation of liquid polymerizable material from the liquid polymerizing mass. Moreover, such evaporation would disrupt the homogeneity of the polymerizable liquid, especially when a syrup of polymer dissolved in monomer is used, and this would result in the production of sheeting having optical distortions. Furthermore, if an inert gas were permitted to flow over the entire length of the unit, evaporation of polymerizable liquid into the gas would saturate the gas with vapor of polymerizable liquid, and this saturated atmosphere would cause pitting of the polymerized sheet at 4 the exit end of the unit. Hence, an inert substantially stagnant atmosphere is maintained above the upper surface of the polymerizing mass, permitting the inert gas to flow only across the upper surface of the substantially polymerized sheet.

The following example illustrates embodiments of the invention in which parts are by weight unless otherwise stated.

An aqueous solution of magnesium chloride was prepared, saturated at about 10 C., and having a specific gravity of about 1.3 at that temperature. Sodium nitrite was added to the solution, the quantity being sufficient to produce a concentration of about 0.001%, based upon the total weight of the aqueous magnesium chloride solution. The resulting solution was poured into a Monel metal tray 5 wide by 28 long by 6" deep, until the solution rose to a depth of about 1 /2".

A syrup of about methyl methacrylate monomer and about 15% of its polymer by weight, having a viscosity of about 15 poises and containing about 0.8% of water, based upon the weight of syrup, and containing 0.3% of alpha,alphaazodiisobutyronitrile, based upon the weight of syrup, was poured on the salt solution through nozzle 3a. If desired, a syrup containing greater or lesser amounts of the polymer dissolved in the monomeric ester may be used and with careful control of polymerization temperature, pure monomer can be used. A pair of retaining belts, 0.05 inch in thickness by 3 inches in width, fabricated from polytetrafluoroethylene, were arranged in a position similar to that shown in Figure 2, immersed in the salt solution to a depth of about 1 /2 and mounted with angle of divergence being about The belts, however, may diverge at an angle between 45 and although the preferred angle is between 80 and 100. These belts, spaced 52" apart, were moved co-directionally at about 7 per hour with the syrup as it advanced with the salt solution, the belts retaining the syrup on the surface of the salt solution so that a sheet of about 50" in width was formed. The methyl methacrylate syrup was distributed onto the surface of magnesium chloride solution at the rate of about cc. per minute from nozzle 3a. having an inside diameter of about The point of distribution of the syrup onto the salt solution was on the center line between syrupretaining belts at a point 3" from the point of divergence of the belts. The salt solution was circulated concurrently with the syrup at about 7 per hour and was cooled and filtered in a continuous fashion. Under the foregoing conditions a polymeric sheet, having a thickness of about was formed.

The glass covering 5 of the Monel metal tray A permitted the maintenance of an atmosphere of nitrogen above the surface of the polymerizing mass. A glass, or other transparent material, that does not absorb unduly ultraviolet light should, of course, be used, for the light bank B above cover A is used to induce polymerization of the monomeric ester. A stream of nitrogen, at the rate of from 5 to 25 liters and preferably about 15 liters per minute, was passed over the polymerized portion of the sheet from a point 4 from the end of the unit, while the salt solution was maintained at a temperature of about 15 C. by a cooling jacket, not shown, beneath the tray. The inlet temperature of the syrup was about 15 C., and during polymerization the temperature of the polymerizing layer rose to a maximum of about 40 C. As the source of polyanemone methacrylate: sheet. was. substantially completely! polymerized; Thesheetingproducedwas of particularly high clarity with". smooth surf aces. and was free of bubbles. Thesheetingtwasclassified as optical quality;

The: example was repeated: except-that a .stagnant; atmosphere of nitrogem was maintained: over: the. entire.- upper surface of the. polymerizeing: mass; The: resulting t sheeting; ;however,. was; found to be highly pitted: .Thex methyl; metlr-.- acrylatevapor in the stagnant: atmosphere attacks the solid sheeting and appears-std. beresponsible for the; pitting; When. such. sheeting isisubsequently. heattreated, ,pitting' in highly accentuated and the resultingssheethasaz frosted appearance. Plasticized sheeting is more vulnerable to pitting thanis unplasticizedsheeting but regardless of:- composition of the sheet produced, surface pitting is eliminated entirely by passing. an: inertv gas, such as nitrogen, over the sheeting as described;

Distribution of the polymerizable liquid should be carried out'in' such a manner that at the time the syrup contacts the retaining means the linear speed of each should be substantially the same. syrup moves rapidly and in all directions across the surface of the salt solution, seeking its ultimate level. The linear speed of the syrup at the point of distribution is about 60-100 times the linear speed of the retaining belts. Enough space should be provided to permit the syrup to level out and gradually slow down to approximately the linear speed of the retaining belts before actually contacting the belts. Accordingly, it has been found that the point of syrup distribution should be sufliciently removed from the syrup retaining belts to insure that the syrup during polymerization, at least, will be moving at approximately the same linear speed as the retaining belts at the time of contact. It follows that the closer the point of distribution to the syrup retaining belts, the greater the difference between the linear speed of the retaining belts and the adjacent portions of syrup, and the greater the shearing forces between the retaining belts and the adjacent portions of syrup. The degree of shear between the retaining belts and syrup determines the number and intensity of longitudinal lines which are formed in the polymerized sheeting. Depending on the intensity of the lines they may or may not be detectable by visual inspection. In most cases these lines are only detectable by observing a shadow pattern of the sheeting.

In general it is preferred to distribute the polymerizable liquid at a point on the center line between the retaining belts. As described above, this point must not be too close to either of them. On the other hand, the point of distribution should not be too far removed from the point of divergence of the belts, otherwise the polymerizable liquid between the point of distribution and the point of divergence of the belts will tend to stagnate. This would result in syrup hold-up and generally would cause non-uniform polymerization. Experience has shown that the point of distribution should be no closer than about 2" from th point of divergence of the retaining belt and no greater than about 12" At the point of syrup distribution the.

- fied as optical sheeting can be i used for various 6;" therefromi. vFurthermore; theepointtofrdistriliu tion does not have tox'bei cm the. center line bet tween the retaining belts so longr'as itris' nottdo close toia retaining-belt. Ingeneral, it: is pre ferredrthat the point of" distributioni be between 3 to 6: inchesfrom the paint off divergence of:

the belts and on the approximate center line between the=belts. Combined with the optimum angle ofdivergence of the belts, distribution of the polymerizable. liquid, preferably-- in. a single stream, at; the optimumpoint results inithe f or mation' of opticalquality sheeting.

Although operation within the optimum condi-.-- tions of thepresentinvention producespolymeric sheeting, especially methyl methacrylate and styrene sheeting, of highest optical qualityg-the so-called optical imperfections 'in'the' sheeting: produced by operation==outside of theoptimumconditions are usually not apparent by visual' inspection. This means thatsheeting'not classi applications, such: as a diffusing means in fluorescent lighting, painted signs; electroplating tanks, and in various ornamental applicationssuch as handbag and umbrella handles, lamp stands; display stands, etc. Optical. sheeting, which isusedfi in aircraftenclosures,.lenses; etc, may be judged by observing azshadow pattern of thevslieeting The flow of polymerizable liquid onto the surface of the aqueous salt solution depends upon the general dimensions of sheet to be formed, that is length, width and thickness. Furthermore, the linear velocity rate of the retaining belts and flow of aqueous salt solution will be balanced with the flow of polymerizable liquid. These variables are adjusted to optimum values depending upon the production rate, the dimen-- sions of the sheeting and general conditions of polymerization. For optimum results the forward movement of the sheet of polymerizing ester, the flow of supporting salt solution and the advance of the belts are synchronized to the same rate of travel which may vary widely. In operating in equipment of the size described, speeds between 5 and 10 feet per hour can be used.

Any inert gas, that is not an inhibitor of polymerization, and which is inert toward the liquid polymerizable organic compound and the resulting polymer thereof, may be used in area D in the process. Besides nitrogen, other inert gases which may be used include helium, neon, argon, etc.

As stated, it is very important that the inert gas be restricted to passing over the upper surface of the substantially solid polymerized sheet. The atmosphere of the inert gas mixed with the vapor of the liquid polymerizable organic compound that stands above the liquid polymerizing mass should be substantially stagnant so that vaporization of the liquid polymerizable material is held to a minimum. This is accomplished by first introducing the inert gas into the unit by means of a perforated tube 1 with discharge openings pointing in the direction of the sheet exit of the unit. Since there is no other outlet for the gas at the inlet end of the unit, inert gas flow is maintained in the direction of the sheet exit. In addition to the proper direction of flow, it is also important that the rate of flow of the gas is not excessive. This should be adjusted in accordance with the general dimensions of the polymerization unit. An excessive rate of flow will tend to cause excessive evaporation of liquid polymerizable compound in the polymerization portion of the unit, resulting in the formation of optical irregularities in the resulting sheeting. On the other hand, a very low rate of vapor flow will be ineffectual in preventing pitting of the sheeting because of the higher concentration of monomeric vapors present.

The invention is particularly useful as applied to the production of methyl methacrylate polymer or styrene polymer sheets, but may be used for the production of sheets on any liquid polymerizable compound. Mixtures of methyl methacrylate or styrene with other polymerizable compounds in lesser proportions may be substituted for the methyl methacrylate or styrene monomers or syrup. Other polymerizable liquids, such as methyl, ethyl, propyl and butyl acrylates and ethacrylates, ethyl, propyl and butyl methacrylates, vinyl chloride, vinylidene chloride, methyl styrene, and the like, are examples of other ethylenically unsaturated compounds to which the process is fully applicable, although sheets of such polymerizable compounds are not in great demand.

I claim:

1. In a process for the preparation of methyl methacrylate polymer sheeting wherein a layer of monomeric and polymeric methyl methacrylate syrup is poured on a moving substantially horizontal surface of an aqueous liquid saturated with a salt, the ester being polymerized thereon to form a sheet, the layer of ester and the salt solution moving co-directionally through an enclosed chamber, the steps which comprise introducing an inert gas into .the enclosed chamber at a position removed from the place at which the monomeric and polymeric methyl methacrylate syrup is poured on the moving surface of the saturated salt solution and beyond the division between the polymerizing mass of syrup and the substantially solid polymerized sheet, maintaining a stagnant vapor in the space above the syrup and a flow of the inert gas in the space above the solid polymerized sheet, whereby optical distortion of the polymeric methyl methacrylate sheet is inhibited.

2. The process of claim 1 in which the inert gas is selected from the group consisting of nitrogen, helium, neon and argon.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,345,013 Soday Mar. 28, 1944 2,537,970 Fields Jan. 16, 1951 2,579,138 Burness et a1 Dec. 18, 1951 

1. IN A PROCESS FOR THE PREPARATION OF METHYL METHACRYLATE POLYMER SHEETING WHEREIN A LAYER OF MONOMERIC AND POLYMERIC METHYL METHACRYLATE SYRUP IS POURED ON A MOVING SUBSTANTIALLY HORIZONTAL SURFACE OF AN AQUEOUS LIQUID SATURATED WITH A SALT, THE ESTER BEING POLYMERIZED THEREON TO FORM A SHEET, THE LAYER OF ESTER AND THE SALT SOLUTION MOVING CO-DIRECTIONALLY THROUGH AN ENCLOSED CHAMBER, THE STEPS WHICH COMPRISE INTRODUCING AN INERT GAS INTO THE ENCLOSED CHAMBER AT A POSITION REMOVED FROM THE PLACE AT WHICH THE MONOMERIC AND POLYMERIC METHYL METHACRYLATE SYRUP IS POURED ON THE MOVING SURFACE OF THE SATURATED SALT SOLUTION AND BEYOND THE DIVISION BETWEEN THE POLYMERIZING MASS OF SYRUP AND THE SUBSTANTIALLY SOLID POLYMERIZED SHEET, MAINTAINING A STAGNANT VAPOR IN THE SPACE ABOVE THE SYRUP AND A FLOW OF THE INERT GAS IN THE SPACE ABOVE THE SOLID POLYMERIZED SHEET, WHEREBY OPTICAL DISTORTION OF THE POLYMERIC METHYL METHACRYLATE SHEET IS INHIBITED. 